The most widely used techniques for measuring the volume of a human body are underwater or hydrostatic weighing and water displacement. However, since both techniques involve subject submersion, they are both inappropriate for use with infants and newborns.
The present invention is directed to a noninvasive apparatus and method to measure volume of an object, such as infants, using the principle of the Helmholtz Resonator. The Helmholtz Resonator consists of a container having a cavity having an enclosed volume of gas which can be caused to resonate acoustically through an opening in the container by the imposition of periodic pressure fluctuations at the opening. The classic equation for the resonance frequency for the Helmholtz Resonator is ##EQU1##
Where c is the speed of sound in the gas, A is the cross-sectional area of the opening, V is the volume of the resonating cavity, l is the length of the neck of the opening and R is the radius of the opening. From this equation, it is seen that the resonance frequency is inversely proportional to the square root of the cavity volume. If the volume of the resonating cavity changes, the resonance frequency will also change. The present invention uses this basic characteristic of the Helmholtz Resonator by measuring the resonance frequency of the cavity in the container before and after an object, such as an infant, is placed inside of the cavity. The difference in the two resonance frequencies can then be calibrated in terms of the total object volume.